how to convert rosrun mux command to a launch file

edit

I have a written several launch files that work. But the format of topic_tools mux with its list of unnamed arguments baffles me. I can make things work with

rosrun topic_tools mux sel_cmdvel joystick/cmd_vel cmd_vel mux:=mux_cmdvel

How can I put this in a launch file? like the following which doesn't work.

<launch>
 <!-- mux_cmdvel installation -->
 <node pkg="topic_tools" type="mux" name="mux_cmdvel" args="sel_cmdvel joystick/cmd_vel cmd_vel">
 </node>
</launch>

In response to Dornhege's request below, I add the following clarification which may or may not help.

I have a launch file which launchs 2 nodes which compute a joystick cmd_vel called “LeafJS/cmd_vel” and another node, leaf_base, which is a base controller and expects an input topic called sel_cmd_vel. I am using mux to subscribe to the LeafJS/cmd_vel topic and output the sel_cmd_vel topic. Of course the robot’s wheels do not respond to joystick commands at this point since mux isn’t running.

After running rosrun topic_tools mux sel_cmd_vel LeafJS/cmd_vel cmd_vel mux:=mux_cmdvel, the robot runs with mux selecting the default input LeafJS/cmd_vel and outputing sel_cmd_vel to the base controller which drives the wheels. So, the mux is apparently working.

Now, the plot thickens…I run rosnode info mux_cmdvel as you requested and get

alex@rocky:~$ rosnode info mux_cmdvel
--------------------------------------------------------------------------------
Node [/mux_cmdvel]
Publications: None

Subscriptions: None

Services: None

cannot contact [/mux_cmdvel]: unknown node

So, I look at rqt_graph which indicates that the mux node is actually called sel_cmd_vel_mux_1380399404626786209

If I rosnode info this node, I get:

alex@rocky:~$ rosnode info sel_cmd_vel_mux_1380399404626786209
--------------------------------------------------------------------------------
Node [/sel_cmd_vel_mux_1380399404626786209]
Publications: 
 * /mux_cmdvel/selected [std_msgs/String]
 * /rosout [rosgraph_msgs/Log]
 * /sel_cmd_vel [geometry_msgs/Twist]

Subscriptions: 
 * /LeafJS/cmd_vel [geometry_msgs/Twist]
 * /cmd_vel [unknown type]

Services: 
 * /mux_cmdvel/delete
 * /mux_cmdvel/list
 * /mux_cmdvel/add
 * /sel_cmd_vel_select
 * /sel_cmd_vel_mux_1380399404626786209/set_logger_level
 * /sel_cmd_vel_mux_1380399404626786209/get_loggers
 * /mux_cmdvel/select


contacting node <a href="http://rocky:49171/">http://rocky:49171/</a> ...
Pid: 4008
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound
    * transport: TCPROS
 * topic: /sel_cmd_vel
    * to: /leaf_base
    * direction: outbound
    * transport: TCPROS
 * topic: /sel_cmd_vel
    * to: /rostopic_4120_1380399530443
    * direction: outbound
    * transport: TCPROS
 * topic: /LeafJS/cmd_vel
    * to: /leaf_teleop_joystick (<a href="http://rocky:44908/">http://rocky:44908/</a>)
    * direction: inbound
    * transport: TCPROS

My rosrun mux… command seems just like the example in the wiki, so I don’t see why it has the long name rather than the simple assignment to mux_cmdvel. But, it is working.

If I do run the mux launch file above, I do get a mux_cmdvel node with the following info:

alex@rocky:~$ rosnode info mux_cmdvel
--------------------------------------------------------------------------------
Node [/mux_cmdvel]
Publications: 
 * /mux_cmdvel/selected [std_msgs/String]
 * /rosout [rosgraph_msgs/Log]

Subscriptions: 
 * /cmd_vel [unknown type]

Services: 
 * /mux_cmdvel/set_logger_level
 * /mux_cmdvel/delete
 * /mux_cmdvel/list
 * /mux_cmdvel/add
 * /LeafJS/cmd_vel_select
 * /mux_cmdvel/select
 * /mux_cmdvel/get_loggers


contacting node <a href="http://rocky:44309/">http://rocky:44309/</a> ...
Pid: 4684
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound
    * transport: TCPROS

which seems to indicate it is launching some kind of default mux with the specified name, but not setting it up per the arguments. So my question remains, how do I specify the arguments so that the mux node is using LeafJS/cmd_vel ... (more)